|Methods of study|
The rocks and fossils
The project is based on microvertebrate fossils which are extracted from the sediment following a standard preparation protocol. Fossils may be seen on the surface of the rock, the fish teeth and scales often being black and shiny, and the reptile bones and teeth often white or brown in colour. In the fissures, teeth are mostly white-yellow to orange in colour. Larger pieces of bone or teeth may be removed from the rock by hand, but most pass to acid digestion. Each rock is checked for visible fossils, which are glued with MowitalTM in ethanol, which dries to form a plastic layer that consolidates and protects the bone from acid digestion.
Rock and fossil processing
The process of preparation follows methods established by Whiteside (1983), and presented in detail by Van den Berg et al. (2012). The rocks are processed in 5% acetic acid solution, with tri-calcium added as a buffer to protect the uncovered bone surface. Mowital is used in preference to ParaloidTM as it provides a strong bond and is more easily reset (Davidson and Alderson, 2009). After 45-60 hours, the rock is thoroughly cleaned with water, and the water and residue are run through a sieve with mesh size 250 μm. All acid digestion is done inside a fume hood to permit safe extraction of fumes.
The rock and residue are then returned to the water, with a drop of soap (sodium hydroxide) to neutralise any remaining acid, and left for a further 45-60 hours, but fungus sometimes grows, and the process is stopped. After cleaning, the rock and residue are run through the acid and neutralising cycle repeatedly until there is no acid left.
The residue is then run through four sieves, with mesh sizes of 1180 μm, 600 μm, 425 μm and 250 μm. The residue is dried on filter paper in a funnel over a small bucket, and, when dry, is brushed into storage boxes labelled according to the source rock and the processing regime. The contents of each box are then tipped onto standard microfossil grids, and hand picked under a binocular microscope. Identifiable specimens are sorted into broad morphotypes.
Key specimens are photographed with a Leica MZ12.5 stereomicroscope equipped with a digital camera. Images are processed using QCapturePro 6.0. For every new specimen and after any modification to the microscope, light and white balance are adjusted. A black background is chosen to increase contrast. Because many of the specimens are teeth, one magnification is used for as many as possible (magn. 1.6 x 10). Adjustments are only made for conodont elements (magn. 4.0 x 10), fish teeth (magn. 4.0 x 10) and specimens that are very large, like sphenodontian jaws (magn. 1.0 x 10).
Measurements are made under the microscope using the built-in measuring device as well as a scale photographed beside the fossil. Measuring teeth is difficult. Tooth height is measured by placing the tooth with the base as horizontal as possible, before measuring the vertical line from the highest portion (tip if present) to the base. Tooth width is the width of the crown at its broadest part.
Identifying specimens and determining faunal composition
In most studies, thousands, sometimes tens of thousands, of specimens are processed and sorted. They are separated into teeth, bones and bone shards from fishes and reptiles, as appropriate. Often the numerous bone shards are beyond recognition.
Microvertebrate collections readily provide information on the relative proportions of the taxa. However, this can be difficult to achieve. Normally, the reworked, older materials are first discarded. Then, decisions are made about how to quantify numbers when some taxa (e.g. archosaurs) are represented by isolated teeth, whereas others (e.g. sphenodontians) are represented mainly by partial jaws to which the teeth are firmly fused. The same is true of fish teeth, where some teeth are simple and usually complete, whereas others may be complex, multicusped structures, and often represented by partial fragments.
Apart from the variable completeness of individual specimens, and their relationship to potential numbers of animals, there are also the problems of numbers of parts and shedding. Simply counting isolated teeth may not reflect true proportions of their owners in the original fauna if different animals have different numbers of teeth or scales. Further, some such as sharks readily shed numerous teeth during their lives, whereas others may have produced far fewer teeth in the same time.
How it works
The project 'At the feet of the dinosaurs' is linked to the Bristol Dinosaur Project and offers students a chance to experience real research. Each student is assigned a site, does field work, works through the fossil collection, identifies specimens, and writes and illustrates the paper.
The work stretches the abilities of the students, encouraging them to think critically about what they are doing, and what they read, to query earlier accounts, identify new stories worth investigating, and to have the persistence to complete an independent project. At first, in reviewing a collection of hundreds or thousands of microvertebrate fossils, it may seem impossible to make sense of what is there. The students learn to identify different morphotypes, sort out the materials into categories, and then to engage with the published literature and established experts to determine the identities of their bones, teeth, and scales.
The tricky part is then to make sense of what they have found. What's new? Does my collection illustrate something that has only been suspected before? How does the site compare with others? Does it illuminate some regional pattern?